1. Field of the Invention
The present invention relates to a process for polymerizing olefins of the formula CH2CHR in which R=H or a C1-C10 alkyl group, and copolymerizing said olefins with alpha olefins of C3-C8 carbons in a slurry or gas phase process using new Ziegler-Natta catalyst systems. More particularly, this invention relates to catalytic homopolymerization of ethylene and copolymerization of ethylene with alpha olefins using a catalyst which contains at least magnesium, aluminum and titanium chemically anchored on a polymeric material. The product polyethylene polymer and copolymers have a density of about 0.91 to 0.97, molecular weight of about 500 to 900,000 grams/mole, a very low level of fines, uniform spherical particles, very good thermal stability and excellent optical properties.
2. Description of the Prior Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
The field of olefin polymerization catalysis has witnessed many remarkable discoveries during the last 50 years. In particular two broad areas of invention stand out. Firstly, the discovery of Ziegler-Natta catalysts in the 1950""s, which are still being used extensively in the polyolefins industry. Secondly, and more recently, the discovery of the highly active metallocene-based catalysts. Since the discoveries of these systems, extensive research work was conducted in order to improve their performance.
However, despite the progress in these areas, there are still certain limitations as recognized by those of ordinary skill in the art. For example, conventional silica supported Ziegler-Natta catalysts often display limited activity, which reflects on the high catalyst residues. On the other hand, heterogeneous metallocene-based catalysts intrinsically possess high activity, though the catalyst precursors and, in particular the cocatalysts required for polymerization, such as aluminoxanes or borane compounds, are very expensive and troublesome in use. Further, another limitation that both catalyst systems share is the lengthy method of preparation and relatively high levels of fines generated in the polymers.
Traditionally, the active components of both Ziegler-Natta and metallocene catalysts are supported on inert carriers to enhance the catalyst productivity and to improve and control the product morphology. Magnesium chloride and silica have predominantly been used for the preparation of supported olefin polymerization catalysts.
U.S. Pat. No. 4,173,547 to Graff describes a supported catalyst prepared by treating a support, for example silica, with both an organoaluminum and an organomagnesium compound. The treated support was then contacted with a tetravalent titanium compound. In a simpler method, U.S. Pat. No. 3,787,384 to Stevens et al. discloses a catalyst prepared by first reacting a silica support with a Grignard reagent and then combining the mixture with a tetravalent titanium compound.
However, procedures typically used for the preparation of suitable magnesium chloride and silica supports such as spray drying or re-crystallization processes are complicated and expensive. Hence, all methods described in the aforementioned patents of catalyst preparation present the inconvenience of being complicated, expensive and do not allow consistency of particle size and particle size distribution. Also, despite the extensive and increasing use of the described supports for Ziegler-Natta catalysts, the support materials themselves have several deficiencies. For example, in the case of silica, high calcination temperatures are required to remove water, which is a common catalyst poison. This represents a significant proportion of the preparation of the catalyst. The use of silica as a support results in the support remaining largely in the product, which can affect product properties, such as optical properties or processability.
Certain polymeric materials have also been used for supporting titanium and magnesium compounds. However, most of the polymeric supports used so far have been based on polystyrene or styrene-divinylbenzene copolymers. U.S. Pat. No. 5,118,648 to Furtek and Gunesin describe a catalyst prepared using styrene-divinylbenzene as a polymeric support. The preparation of the catalyst was carried out by suspending the polymeric support in a solution of a magnesium dihalide or a magnesium compound capable of being transformed into a magnesium dihalide, for example, by titanium tetrachloride treatment, and subsequently evaporating the solvent. Hence, the active catalyst components were deposited on the polymeric support by physical impregnation. Other physical impregnation methods include those described in U.S. Pat. No. 4,568,730 to Graves whereby polymer resins of styrene-divinylbenzene are partially softened and the active catalyst components are homogeneously mixed in the resin to form a mass, which was subsequently pelletized or extruded into catalyst particles. However, the activity of the above-described polymer supported catalysts is not higher than that of metal oxide supported Ziegler-Natta catalysts.
Polypropylene and polyethylene have also found use as polymeric supports, where the polymeric material is typically ground with the catalyst components, which represents a difficult and complicated catalyst preparation procedure. In addition, there remains a significant concern as to the ability of the support material to retain the active species, deposited by physical impregnation, during polymerization conditions and thus generate, for example, fines. Hsu et al, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 32,2135 (1994), have used poly(ethylene-co-acrylic acid) as a support for Ziegler-Natta catalysts and the catalyst activity was found to be similar to that of the magnesium chloride supported catalyst.
It is an object of the invention to overcome the above-identified deficiencies.
It is another object of the invention to provide a process for the production of olefin homopolymers and copolymers, especially ethylene homopolymers and copolymers.
It is a further object of the invention to provide polymers having a very low level of fines. The foregoing and other objects and advantages of the invention will be set forth in or become apparent from the following description.
This present invention provides a process of making ethylene polymers and ethylene/alpha olefins (C3-C8) copolymers in slurry or gas phase, having a wide density range of about 0.91 to 0.97 grams/cm3 and weight average molecular weight (Mw) of about 500 to 900,000 grams/mole and molecular weight distribution range of 2 to 10. The product ethylene homopolymers and copolymers have a uniform spherical particle morphology, very low level of fines and catalyst residues, improved thermal stability, excellent optical and better environmental stress cracking resistance (ESCR) than products made with other catalysts heretofore known in the art. The ethylene homopolymers and copolymers can be produced with the very highly active new Ziegler-Natta catalyst systems including at least a transition metal compound, a magnesium compound and an aluminum compound, chemically anchored on polymeric particles having labile active sites.
As a result of the present invention olefin and especially polyethylene polymers and copolymers are provided which have a density of from about 0.91 to about 0.97 grams/cm3 molecular weight of from about 500 to about 900,000 grams/mole, a very low level of fines, uniform spherical particles, very good thermal stability and excellent optical properties. Additionally, by using the process of the present invention copolymers of ethylene with alpha olefins are obtained at a productivity xe2x89xa71,000,000 gm PE/gm Ti.
The ethylene homopolymers and copolymers which may be prepared by the process of the present invention can have a wide density range of from about 0.91 to about 0.97 grams/cm3. The process of the present invention provides polyolefins, and preferably high density polyethylene and linear low density polyethylene. The density of the polymer at a given melt index, can be regulated by the amount of the alpha olefin (C3 to C8) comonomer used. The amount of alpha olefin comonomer needed to achieve the same density is varied according to the type of comonomer used. These alpha olefins can include propylene, 1-butene, 1-pentene, 4-methyl 1-pentene, 1-hexene, 1-heptene and 1-octene.
The average molecular weight (Mw) grams/mole of the polymers obtained in accordance with this invention ranges from 500 to 900,000 grams/mole or higher, preferably from 10,000 to 750,000 grams/mole, depending on the amount of hydrogen used, the polymerization temperature and the polymer density attained. The homopolymers and copolymers of the present invention have a melt index (MI) range of more than 0 and up to 100, preferably between 0.3 to 50.
The polydispersities, i.e., molecular weight distribution (MWD) of the produced polymers expressed as molecular weight/number average molecular weight of the polymer (Mw/Mn), is in the range of about 2 to 10. The polymer melt flow ratio (MFR) is another means of indicating MWD. The polymers of the present invention have an MFR in the range of about 15 to 60, preferably 20 to 40. Polymers having such a wide range of MFR are capable of being used in molding and film applications.
The polymers of the present invention are granular materials, uniform and spherical particles with an average particle size of about 0.1 to 4 mm in diameter, and a very low level of fines. The bulk density of the polymer ranges from 0.20 to 0.35 g/cm3.
The solid catalyst component (catalyst precursor) used in the present invention contains at least a transition metal compound, a magnesium compound, an aluminum compound and a polymeric material having a mean particle diameter of 5 to 1000 xcexcm, a pore volume of 0.1 cm3/g or above and a pore diameter of from 20 to 10,000 Angstrom, preferably from 500 xc3x85 to 10,000 xc3x85 and a surface area of from 0.1 m2/gm to 100 m2/gm, preferably from 0.2 m2/gm to 15 m2/gm.
The transition metal compound used for the synthesis of the solid catalyst component in the invention is represented by the general formula M(OR1)nX4xe2x88x92n, wherein M represents a transition metal of Group IVA, VA, VIA, VIIA or VIII of the Periodic Table of the Elements, R1 represents a alkyl group having 1 to 20 carbon atoms, X represents a halogen atom and n represents a number satisfying 0xe2x89xa6nxe2x89xa64. Nonlimiting examples of the transition metal are titanium, vanadium, or zirconium. Examples of R1 include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl and the like.
Preferred examples of the above mentioned transition metal compounds include the following: titanium tetrachloride, methoxy titanium trichloride, dimethoxy titanium dichloride, ethoxy titanium trichloride, diethoxy titanium dichloride, propoxy titanium trichloride, dipropoxy titanium dichloride, butoxy titanium trichloride, butoxy titanium dichloride, vanadium tdichloride, vanadium tetrachloride, vanadium oxytrichloride, and zirconium tetrachloride.
The magnesium compound used for the catalyst synthesis in the invention include Grignard compounds represented by the general formula R2MgX, wherein R2 is an alkyl group of 1 to 20 carbon atoms and X is a halogen atom. Other preferred magnesium compounds are represented by the general formula R3R4Mg, wherein R3 and R4 are each an alkyl group of 1 to 20 carbon atoms.
Preferred examples of the above mentioned magnesium compounds include the following: diethylmagnesium, dibutylmagnesium, butylethylmagnesium, dihexylmagnesium, butyloctylmagnesium, ethylmagnesium chloride, butylmagnesium chloride, hexylmagnesium chloride and the like and mixtures thereof.
These magnesium compounds described above may also be used in catalyst preparation as a mixture with an organoaluminum compound. Examples of organoaluminum compounds include trialkylaluminium such as trimethylaluminum triethylaluminum, triisobutylaluminum, trihexylaluminum and the like and mixtures thereof, and alkylalumoxanes such as methylalumoxane, ethylalumoxane and the like. The mixture of the magnesium compound and the organoaluminum compound in this invention can be used with a Mg:Al molar ratio of 99:1 to 50:50, and preferably 98:2 to 80:20 and more preferably 96:4 to 85:15.
The polymer particles used as supports in the present invention are in the form of distinct spherical particles, on which the active catalyst component is chemically bonded, wherein the ratio of active catalyst component to polymeric support is less than 3% by weight, preferably less than 1% by weight, more preferably less than 0.7% by weight. In contrast, catalysts prepared in the prior art using polymeric materials relied on physical impregnation of the catalyst active sites on the polymeric materials.
The polymer particles used in the present invention have a spherical shape with a particle diameter of 5 to 800 xcexcm, preferably 10 to 600 xcexcm, and more preferably 15 to 500 xcexcm, a pore diameter of 20 to 10,000 Angstroms, preferably from 500 xc3x85 to 10,000 xc3x85, surface area of from 0.1 m2/gm to 100 m2/gm, preferably from 0.2 m2/gm to 15 m2/gm, a pore volume of 0.1 cm3/g or above, preferably 0.2 cm3/g or above, and a molecular weight in the range of 5,000 to 200,000 g/mole. Uniformity of particle size is not critical and in fact catalyst supports having nonuniform particle sizes are preferred. By way of example and not as a limitation, for a catalyst support having a median particle size of 65 microns, it is preferred that at least 10% of the support particles have a diameter of greater than 85 microns, and at least 10% of the support particles have a diameter of less than 45 microns.
Examples of the polymeric supports used in the catalyst preparation of the present invention include polymer beads made of thermoplastic polymers. Polymer supports made of polyvinylchloride are preferred, and non-cross linked polyvinylchloride particles are most preferred.
The polymer particles used in the present invention have surface active sites such as labile chlorine atoms. Preferably, these active sites are reacted stoichiometrically with the organometallic compound, namely a magnesium containing compound and/or an aluminum containing compound.
The use of the polymer particles mentioned in the catalyst preparation of the invention offers significant advantages over traditional olefin polymerization catalysts using supports such as silica or magnesium chloride. In comparison to the silica supported catalyst, the polymer particles described in catalyst preparation of the invention require no high temperature and prolonged dehydration steps prior to their use in catalyst synthesis, thereby simplifying the synthesis process and thus reducing the overall cost of catalyst preparation. Furthermore, the cost of the polymeric support used in the present invention is substantially cheaper than silica or magnesium chloride supports. In addition, the catalyst in the present invention uses significantly lower levels of catalyst components for catalyst precursor preparation than silica or magnesium chloride supported catalysts. Also, the catalyst in the present invention is more active than conventional silica or magnesium supported Ziegler-Natta catalysts and supported metallocene catalysts. It has been unexpectedly found that the catalyst compositions of the present invention have an activity of more than 60,000 g polyethylene per mmol of titanium per 100 psi per hour, thereby providing polymers of superior clarity.
According to one embodiment of the present invention, polyvinyl chloride is used in the synthesis of the solid catalyst component. The synthesis of the solid catalyst component in the present invention involves introducing the polymeric material described above into a vessel and adding a diluent. Suitable diluents include isopentane, hexane, cyclohexane, heptane, isooctane and pentamethylheptane and mixtures thereof. The polymeric material is treated with either a magnesium compound described above or a mix of a magnesium compound and aluminum compound of the type described above at a temperature in the range of 20xc2x0 C. to 150xc2x0 C., preferably 50xc2x0 C. to 110xc2x0 C. The ratio of organometallic compound to the polymer support can be in the range of 0.05 mmol to 20 mmol per gram polymer, preferably 0.1 mmol to 10 mmol per gram polymer, and more preferably 0.2 mmol to 2 mmol per gram polymer.
The magnesium or magnesium-aluminum modified polymeric material is then treated with a transition metal compound of the type described above at a temperature in the range of 20xc2x0 C. to 150xc2x0 C., preferably 50xc2x0 C. to 110xc2x0 C. According to this invention, TiCl4, TiCl3, Ti(OC2H5)3Cl, VCl4, VOCl3, ZrCl4, ZrCl3(OC2H5) are preferred transition metal compounds, TiCl4, and ZrCl4 are more preferred. The produced solid catalyst component is washed with a suitable solvent such isopentane, hexane, cyclohexane, heptane, isooctane and pentamethylheptane, preferably isopentane or hexane. The solid catalyst component is dried using a nitrogen purge at a temperature in the range of 20xc2x0 C. to 100xc2x0 C., preferably 30xc2x0 C. to 80xc2x0 C.
The free-flowing solid particulate catalyst is activated with suitable activators, also known as cocatalysts or catalyst promoters. The activation process can be a one step in which the catalyst is fully activated in the reactor, or two steps, in which the catalyst is partially activated outside the reactor and the complete activation occurs inside the reactor. The preferred compounds for activation of the solid catalyst component are organoaluminum compounds.
The organoaluminum compounds which can be used as activators in the present invention along with the solid catalyst component are represented by the general formulas R5nAlX3xe2x88x92n or
R6R7Alxe2x80x94Oxe2x80x94AlR8R9, where R5, R6, R7, R8, and R9, each represent an alkyl group having 1 to 20 carbon atoms, such as a hydrocarbon; X represents a halogen atom or an alkyl group; and n represents a number satisfying 0xe2x89xa6nxe2x89xa63. Illustrative but not limiting examples of organoaluminum compounds include triethylaluminum (TEAL), triisobutylaluminum (TIBA), tri n-hexylaluminum (TnHAL), diethylaluminum chloride, methylalumoxane, ethylalumoxane, and mixtures thereof. The organoaluminum compound in this invention can be used in the range of from 1 to 1500 moles per one mole of transition metal in the said catalyst, and more preferably in the range of 10 to 800 moles per one mole of transition metal.